What happens when mammals have two mothers? In the case of mice, it seems that they live longer than mice with one male and one female parent. It could help explain why women typically live longer than men.

Researchers in Japan manipulated mouse eggs to grow mice that were bi-maternal, having genetic material from two female parents, but no male parent. They then studied these mice alongside mice with one male and one female parent. The bi-maternal mice and the control mice were kept in the same conditions and fed the same diet, but the bi-maternal mice were significantly smaller and lighter, seemed to have better immune systems, and had an average lifespan 186 days longer than the control mice.

Tomohiro Kono from the Tokyo University of Agriculture, one of the researchers, believes that the bi-maternal mice might have lived longer because of the absence of a gene mice inherit from their fathers, though further study is needed:

"We believe that the most likely reason for the differences in longevity relates to the repression of a gene called Rasgrf1 in the BM mice. This gene normally expresses from the paternally inherited chromosome and is an imprinted gene on chromosome 9 associated with post-natal growth. Thus far, it's not clear whether Rasgrf1 is definitively associated with mouse longevity, but it is one of the strong candidates for a responsible gene. Furthermore, we cannot eliminate the possibility that other, unknown genes that rely on their paternal inheritance to function normally may be responsible for the extended longevity of the BM mice."

If Rasgrf1 is responsible for mouse longevity, it could be responsible for human longevity as well, and could go a long way toward explaining why women tend to live longer than men.